The present invention relates to a ceramic porous substrate with a seed crystal-containing layer used for manufacturing a zeolite membrane, a zeolite membrane obtained by the use of the substrate, and a method for manufacturing the zeolite membrane.
Zeolite is a kind of silicate having pores in the crystal structure, and many kinds (type) having different crystal structures (pore structures) such as LTA, MFI, MOR, AFI, FER, FAU, and DDR are present. Each of these kinds of zeolite has its own adsorbability, catalyst performance, ion exchangeability, and the like, based on a crystal structure and a chemical composition of each kind, and these zeolites are used in various kinds of fields such as adsorbing materials, catalysts (catalyst carriers), and ion exchangers.
In particular, in recent years, gas separation has been conducted by the use of pores peculiar to zeolite. For example, a zeolite membrane of a DDR (Deca-Dodecasil 3R) type zeolite (see, e.g., W. M. Meier, D. H. Olson, Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier (1996)) containing silica (SiO2) as the main component, containing oxygen 8-membered ring, and having relatively small pores (pore size of 4.4×3.6 angstrom) among various kinds of zeolite in the crystal structure is used as a separation membrane to allow only carbon dioxide (CO2) gas or the like to selectively permeate the membrane to separate the carbon dioxide (CO2) gas or the like from the mixed gas.
Upon gas separation as described above, there is generally used a porous substrate of ceramic or the like with a zeolite membrane formed on a surface thereof from the viewpoint of improving mechanical strength while securing a gas permeation amount.
Such a zeolite membrane can be obtained, for example, by allowing a zeolite powder for a seed crystal to adhere to a surface of a porous substrate of ceramic or the like and subjecting the substrate with a zeolite adhering thereto to a heat treatment (hydrothermal synthesis of zeolite) in a state that the substrate is immersed in a raw material solution for synthesizing a zeolite containing a structure-determining agent, water, a silicon source, and the like, to grow a zeolite powder (seed crystal) to form a zeolite membrane, followed by a thermal treatment in an oxygen-containing atmosphere (see, e.g., JP-A-2003-159518).
However, by the aforementioned method, a defect such as a crack is sometimes generated in the zeolite membrane formed on a surface of the porous substrate upon the thermal treatment (conducted in order to burn out the structure-determining agent remaining in the synthesized zeolite) or upon its use for gas separation or the like even if a defect is not generated while manufacturing the zeolite membrane. Such a defect is not preferable in that the gas-separation function in the gas separator is remarkably deteriorated.
In addition, since the zeolite powder (seed crystal) allowed to adhere to a surface of the porous substrate easily falls off from the porous substrate, any air-spraying or washing for the purpose of removing a foreign substance is impossible prior to hydrothermal synthesis of zeolite, and the processing of the substrate after adhesion of zeolite also becomes impossible.
Further, there is a problem of having difficulty in handling because it is necessary to carefully handle the substrate in case the zeolite powder falls from the surface of the substrate upon immersing the substrate, with the zeolite powder adhering thereto, in a raw material solution for synthesizing a zeolite.